Now most people who do that will end up terrible writers. But when someone like Eliezer does it, the results are spectacular.
You could have found a more convincing example.
The objective metrics of quality of literature are hard to come by, but HPMOR does suffer from quite many, many stereotypical sins of fanfic / bad genre writing and makes a tiresome read. (One that I found especially grating and made me finally drop the story altogether could be described as "my main character is not OP because I set up these plainly arbitrary obstacles as a 'balance' ". Please, no. There's more to writing enjoyable, interesting characters in meaningful stories than such naive "balancing". The preferable end result is a piece of fiction that has something more going in them than surface-level entertainment plot which is amenable to measured in terms such as "is my character OP".)
However, I did not register an account just to lambaste Eliezer's fiction. Here's a couple of points that hopefully tie this comment to the main thread of discussion (so that this contribution provides some signal instead of pure noise):
Taking a year-long course in lit or at least some input from the tradition of literature might have improved Eliezer's writing. A class isn't the only way to attain that input, but it certainly helps in finding out if you have missed something vital in your self-study. (After finding out those pieces of information you are free to judge and dismiss them, too, if you want, but you are now dismissing that information with the knowledge that it exists, which is prone to make your act of dismissal more intelligent and productive.)
I don't have exhaustive collection of biographies at hand, but I believe the general trend in "successful writing" is that the significant portion (probably majority) of successful writers (including, but not limited to, authors included in the Western canon as creators of "good literature"), read a lot, wrote a lot, and had a lot of corrective input to improve their writing during their writing careers. Actually, I wouldn't be surprised if prior to starting a writing career, they already had lots of corrective input to improve their general storytelling skills before they even started considering writing. The input came in the form of teachers, friends who reviewed their manuscripts in process (or commented on their stories), how popular their first published drabbles were, and (often most importantly) the input given by the publishing house editors whose job is to improve writing of authors who submit their fiction for publishing.
Eliezer did have a lot of writing practice and feedback since childhood, but with a peculiar audience (transhumanists on the net).
LW has a problem. Openly or covertly, many posts here promote the idea that a rational person ought to be able to self-improve on their own. Some of it comes from Eliezer's refusal to attend college (and Luke dropping out of his bachelors, etc). Some of it comes from our concept of rationality, that all agents can be approximated as perfect utility maximizers with a bunch of nonessential bugs. Some of it is due to our psychological makeup and introversion. Some of it comes from trying to tackle hard problems that aren't well understood anywhere else. And some of it is just the plain old meme of heroism and forging your own way.